Communications Systems widely adopt error correction coding techniques to combat with error introduced by interference and noise. FIG. 1 illustrates a typical communication system with error correction techniques which comprises an error correction encoder 100 at transmitter and a decoder 110 at receiver. The class of error control codes, referred to as turbo codes, offers significant coding gain for power limited communications channels.
Turbo codes are decoded by iterative decoding algorithms. It belongs to a family of iterative decoding algorithms applicable to LDPC code, turbo product code, parallel concatenated convolutional code, serial concatenated convolutional code and etc. During each iteration, the decoder improves the reliability of the information bits. In order to reach a sufficient decoding performance, usually a fixed number of iteration is used. However, in many cases, the correct decoding results can be generated with only a few iterations. Excessive iteration does not help to improve the performance further and only consumes extra power which is a critical resource in mobile devices.
Many early stop algorithms have been proposed to reduce the power consumption for turbo decoding. These algorithm claims to have good performance but these studies did not looked into practical scenarios where the code rate is high. For example, a hard-decision comparison based algorithm has been proposed in [1]. This algorithm compares hard-decisions of information bits from the current iteration and that of the previous iteration. The decoding is stopped once the hard-decisions from the previous iteration matches with those of the current iteration. This algorithm introduces large performance loss in high SNR channel conditions especially with the combination of high code rate. This made the algorithm difficult to be implemented into a practical communication system since the practical communication system like HSDPA usually applies high code rate when signal to noise ration is sufficient. Another early stop algorithm [1] has been proposed which computes the minimum log likelihood ratio (LLR) of decoded output bits. If the minimum value of LLR of all bits exceeds the certain threshold, the decoding is stopped. However, a large block error rate (BLER) performance loss is reported when channel SNR is high. Among various early stop algorithms proposed, no sufficient investigation in the literature has been found to explore the relationship between the BLER performance loss and code rate.
[1] A. Matache, S. Dolinar and F. Pollara, “Stopping Rules for Turbo Decoders”, TMO Progress Report 42-142, Jet Propulsion Laboratory, Aug. 15, 2000.